The invention relates to a digital facsimile transmission system for effectively transmitting newspaper pages and the like, which include screened pictures.
In a conventional facsimile system, a picture to be transmitted consisting of a number of picture elements (pels) is optically scanned to produce a two-level (black and white) picture signal representing a chain of lines each composed of a plurality of pels and to convert the picture signal into a corresponding digital signal, which is composed of a train of binary codes corresponding to the pels. More specifically, within a facsimile transmitter, a coder encodes a voltage proportional in amplitude to the level of brightness of a pel. The encoded voltage is then transmitted to a receiver, where it is decoded to reproduce the original picture.
One coding system to reduce the number of bits to be transmitted is described in an article entitled "Image Data Compression by Predictive Coding I: Prediction Algorithms" by H. Kobayashi and L. R. Bahl, published in IBM Journal of Research and Development, Vol. 18, No. 2, March issue, 1974, pp. 164-171 (Reference 1). In this system, the prediction of a pel code is made on the basis of binary codes representative of neighboring pels including the preceding line. In more particular, the difference between the predicted pel code and the present pel code is taken as a prediction error code (error code), which is then coded for transmission to a receiver. For this reason, the system is suitable for coding of documents such as those including letters and photographs for offset printing, because they have a strong correlation between adjacent pels on a scanning line. However, the system cannot achieve the above-mentioned reduction of the bit number to be transmitted when applied to the coding of newspaper pages including printed character portions and screened picture portions, the latter having a strong correlation between adjacent screens rather than adjacent pels.
A digital facsimile transmission of newspaper pages is discussed in a paper titled "Coding of Two-Tone Images" by Thomas S. Huang published in IEEE Transactions on Communications, Vol. COM-25, No. 11 (November issue, 1977), pp. 1406-1424 (Reference 4). However, the Huang paper does not give any structural details therefor.